1. Field of the Invention
The invention relates to electrostatic microphones comprising a capsule housing in which a diaphragm and a rigid electrode as well as, optionally, a friction pill and an electric circuit on a printed circuit board are arranged.
2. Description of the Related Art
The invention thus relates to electroacoustic transducers which operate as sound receivers, are used as microphone capsules, and operate according to the electrostatic principle. Such transducers have, independent of their physical operating mode, a diaphragm which is exposed to a field of sound and is directly excited by it to vibrate.
The electrodes of the electrostatic transducer are an elastic, taut diaphragm and a stationary (rigid) electrode which is usually referred to simply as electrode. Both together form a capacitor whose electrical capacitance changes as a result of pressure fluctuations of the field of sound. Since an electric field is built up between the electrodes of the electrostatic transducer, it is possible to convert the capacitance changes of the transducer by means of an amplifier arranged downstream into electrical voltage changes.
Electrostatic capsules can be divided into two groups with respect to the type of application of the electrical field between its electrodes:
1. Electrostatic capsules in which the charges which generate the electrical field are applied by means of an externally supplied voltage (polarization voltage): capacitor capsules.
2. Electrostatic capsules in which the electrical charge is “frozen” on the electrode or diaphragm so that in this way an externally applied voltage is obsolete: electret capsule.
Such capsules are used in a series of applications in a an increasingly miniaturized form; reference is being had in this respect to mobile telephones or handsfree communication devices in vehicles and the like. The increasing miniaturization, in particular, in connection with the mass production of such capsules, requires the assembly as well as the principal configuration to be as economical as possible.
According to the prior art both capsule variants have the same type of configuration in common which is illustrated in FIG. 1. A diaphragm ring 2 is arranged in the capsule housing 1 on which a diaphragm 3 has been pretensioned and fastened. The diaphragm ring 2 must have a certain thickness because its task is to maintain the diaphragm 3 in the pretensioned state. This can be realized only with a thickness of the diaphragm ring starting approximately at 0.7 mm up to 2 mm. Moreover, the diaphragm together with the diaphragm ring should form a unit which is sufficiently robust in order to be processed automatically or manually. A spacer ring of material 4 having excellent insulating properties is introduced and placed onto the diaphragm ring.
By means of this spacer ring, the diaphragm and a rigid electrode 5 are maintained at a fixed distance of a few 10 μm. The electrode 5 which is placed onto the spacer ring 4 in the capsule housing forms the second electrode of the capacitor. It is manufactured of an electrically conducting material and has a perforation.
An acoustic friction 6 is arranged on the electrode 5. It is conventionally produced of plastic material by an injection molding process and has a hole or opening which is covered or closed by a porous material. The acoustic friction 6 serves for acoustic tuning of the microphone capsule relative to the frequency response curve of the output level and the pick-up characteristic of the capsule. The microphone capsule is closed at the rear by an electronic printed circuit 7 on which the electronic components which are necessary for the function of the capsule are arranged.
All of the components arranged in the described sequence on the diaphragm have openings in order to allow the sound to impact on the diaphragm also from the backside of the capsule, which is required for the acoustic tuning of the capsule including providing the desired directional dependency of the pick-up characteristic of the capsule.